1. Field of the Invention
This invention relates to a fail-safe circuit of a fixed speed traveling apparatus for automobile.
2. Description of the Prior Art
One type of fixed speed traveling apparatus for automobile is widely used to control the traveling car speed always at a preset speed automatically by storing the preset speed, and it generally possesses a resume switch function which once cancels the fixed speed traveling control when the brake is applied during fixed speed traveling and returns to the same preset car speed or not depending on the judgement of the driver.
In a system where the cancel signal is generated from a stop lamp switch, since it is presumed that a microprocesser or central processing unit(referred to as CPU) does not accomplish cancel operation normally when the CPU receives the cancel signal, its safety is enhanced by combining with a hardware circuit as its backup use.
A typical conventional prior art is shown in FIG. 1, in which there are provided a fixed speed traveling control apparatus 1, CPU 11 as its control center, and a fail-safe circuit 12 for backup use. This fail-safe circuit 12 is often integrated into the custom integrated circuit(referred to as IC) together with power source control circuit and others of CPU 11. The CPU 11 possesses a program for fixed speed traveling control, and when a voltage +B is applied from a stop lamp switch A(normally OFF) which is turned ON when the brake pedal is stepped on, it is used as cancel signal C1, and the fixed speed traveling control is interrupted. This switch A is provided for lighting up the stop lamps L in the rear part of the car.
The fixed speed traveling control is effected by varying the value of engine negative pressure, namely intake pressure of a intake manifold to be introduced into an actuator(not shown) for control of throttle opening degree through a control valve 22. As a result, a duty control signal b generated by prececution of the fixed speed traveling program of CPU 11 is delivered, and is supplied into transistor Tr1 through OR gate G1, and this transistor passes a pulsive current to the control valve 22. As the duty of this current varies depending on the control signal b, the opening degree of the control valve 22 varies, so that the negative pressure introduced to the actuator changes to vary the throttle opening degree.
A release valve 21 is provided for ON/OFF control to feed or not to feed atmosphere into the actuator. Therefore, during fixed speed traveling control, the release valve 21 is closed to shut off the atmosphere. However, when a cancel signal C1 is fed, the CPU 11 turns the release signal a to high level to turn OFF the transistor Tr2, and power supply to the release valve 21 is cut off, and this valve is opened. Since there are also other cancel signals, the OR gate G2 supplies, including other cancel signal C0, to the base of transistor Tr2. In order to nullify the change of control output b in this period, cancel signals C0, C1 are fed also to the OR gate G1.
In this kind of fixed speed traveling control apparatus, it is sometimes predicted that the fixed speed traveling control is not be interrupted in the case where the cancel signal C1 is fed to the CPU 11 due to program runaway or other cause, and to avoid this, the cancel signal C1 is also directly applied to the transistor Tr2, from the hardware of cancel circuit 12 through OR gate G2. Besides, the current supply into the release valve 21 is forced to be cut off by stop lamp switch B of normally ON type which acts reversely in cooperation with the switch A.
Thus, when it is attempted to guarantee the cancellation of fixed speed control by double or triple means, there arises a new problem. That is, when the brake pedal is stepped on while one of the two stop switches A, B, for example A is defective, the other stop switch B may be put in action. In this case, since cancel signal C1 is not generated, the CPU 11 does not interrupt fixed speed traveling control. Therefore, when stepping off the brake pedal and the switch B is turned ON again, traveling is resumed, from this point, at the set car speed, which is not preferable. This is because a driver believes that, during a fixed speed traveling, when the brake pedal is stepped on, the fixed speed traveling is canceled, and that it is not restored unless resume operation is effected. In such a case, generally, as the actual car speed slows down, difference from the set car speed is widening, and the control duty is approaching 100%, which means a sudden acceleration. Accordingly, this speed change will confuse the driver. It is essential to overcome the problems hereinbefore.